1. Field of the Invention
The present invention generally relates to a copying method, and more particularly, to a method of copying a large number of documents in an electrophotographic apparatus having therein a photosensitive member with an amorphous silicon photosensitive layer.
2. Description of the Prior Art
An electrophotographic apparatus generally accommodates the main eraser to be used as a static eraser. The main eraser applies light to the surface of a photosensitive member after transfer and cleaning to erase the remaining electric charge so that the subsequent electrification may be uniformly executed.
In addition to the main eraser, a pre-transfer eraser is occasionally installed at a location after development and before transfer. The pre-transfer eraser applies light to the surface of the photosensitive member to lower the electric potential of a visible image to which toner adheres, thus resulting in weakened toner adsorptivity and enhanced transfer properties of the visible image.
The installation of the pre-transfer eraser is further advantageous in that a transfer sheet can be readily separated from the photosensitive member, since a setting value of transfer current can be lowered.
The amount of light emitted from the aforementioned erasers, the wavelength thereof and the like have a close connection with the properties of the photosensitive member, and therefore, they are required to be determined in accordance with the kind of the photosensitive member to be used or other factors.
The photosensitive member primarily composed of amorphous silicon recently attracts attention as the photosensitive member superior in resistance to abrasion, light sensitivity, resistance to heat, non-environmental pollution and the like.
However, such a photosensitive member is disadvantageous in that carrier tends to be trapped in a portion to which light is applied during image exposure. Because of this, the surface potential of the portion to which the light has previously been applied undesirably lowers at the time of subsequent electrification, thus causing an image memory, that is to say, a latent image left in the photosensitive member after development. This problem becomes particularly conspicuous when a light source for exposure use emits light having a relatively long wavelength. Even when, for example, a halogen lamp having therein a filter for eliminating long-wavelength component is employed as the light source, the long-wavelength component slightly passing through the filter, in cooperation with other factors, cause the image memory particularly when such a lamp is repeatedly used.
Accordingly, the light source of the main eraser is conventionally so selected as to emit light having plenty of the long-wavelength component which can readily enter the inside of a photosensitive layer. Because, it is well known that the long-wavelength component can release the trapped potential for elimination of the image memory.
However, the amount of light to be emitted from the main eraser is required to be increased for complete elimination of the image memory. This fact causes fatigue of the photosensitive layer which makes it difficult to raise the surface potential of the photosensitive layer, thus resulting in inferior image quality.
On the other hand, it is known that the amount of light to be emitted from the main eraser can be reduced by the pre-transfer eraser employing therein a light source containing the long-wavelength component. It is also known that a charger for erasing use can be used in place of the pre-transfer eraser so that any image memory may not be produced.
However, when the light containing the long-wavelength component is emitted from the pre-transfer eraser, the amount of light to be applied to the visible image portion having thereon the toner greatly differs from that to be applied to the other portion. This fact produces a great difference in the degree of light fatigue of the photosensitive layer, thus occasionally causing the image memory.
If the charger for erasing use is provided, it requires an additional space therefor. Furthermore, since the charger is provided immediately downstream of a developing device, it tends to be soiled by the toner or the like and to cause abnormal discharge.